1. Field of the Invention
The present invention relates to a positioning device for positioning an object.
2. Description of the Related Art
Positioning devices that are capable of accurately displacing system components or objects are employed in numerous technologies, including, for example, lithographic fabrication processes. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may be used to generate a desired circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In order to project the pattern onto the appropriate portion of the substrate, both short stroke accurate positioning and large stroke displacements of the object table are generally applied. In general, the object table, holder, or stage provided with the patterning device requires large displacements in only one direction while the table, holder, or stage provided with the substrate usually requires large displacements in a plane.
Usually, the object table provided with the patterning device or the substrate is connected to a first drive unit comprising a plurality of actuators or linear motors. These actuators or motors allow accurate displacement of the object over a small range (˜1 mm). In many applications, these actuators or linear motors are contactless electromagnetic actuators or motors. The first drive unit is usually mounted on a second drive unit that allows large displacements in at least one direction. This second drive unit may, as an example, comprise of a linear motor or a planar motor.
In such an arrangement, both the first and second drive units have to be energized in order to displace the object table. To illustrate how this is typically accomplished, consider the following conventional drive arrangement.
A first drive unit contains a plurality of electromagnetic actuators, each actuator comprising a first part connected to the object table and a second part that is mounted on a second drive unit. The second drive unit is constructed to displace the first drive unit (together with the object table) in a first horizontal direction (Y-direction), that corresponds to the scanning direction in this example. One of the electromagnetic actuators can generate a force in the Y-direction. The stroke of the electromagnetic actuators of the first drive is limited to only a few mm while the second drive unit enables displacements of ˜0.5 m.
Such an arrangement could be applied in a so-called scanner to position the patterning device relative to the projection system. The scanning operation requires that the patterning device, mounted to the object table, is displaced along the Y-direction. This displacement process can be approximated by three different phases: an acceleration phase, a constant velocity phase and a deceleration phase.
During the first and third phase, both the first drive unit and the second drive unit have to be powered in order accelerate or decelerate the object table in the Y-direction. This is due to the fact that the object table is not rigidly attached to the second drive unit but is positioned relative to the second drive unit by means of the contactless actuators of the first drive unit. Therefore, the first drive unit has to be designed in such a manner that the generated force is sufficient to accelerate the object table with the first parts of the different electromagnetic actuators attached to it. Typical values of the required force in the scanning direction for the first drive unit may be >200 N. Generating this force may result in a significant amount of dissipation in the current carrying coils of the electromagnetic actuators.
During the second phase of the displacement process, i.e. the constant velocity phase, the main objective is the accurate positioning of the object table. In a lithographic apparatus, this phase corresponds to the exposure phase during which the pattern on the patterning device is projected onto the substrate. The patterning device and the substrate have to move synchronously with a nanometer accuracy, in order to project the pattern on the appropriate part of the substrate. In order to meet this accuracy, the requirements of the actuators of the first drive unit are severe with respect to stiffness, dynamic response, damping, etc.